1. Field of the Invention
The present invention relates to a display device, and more particularly to a technique which is effectively applicable to an organic EL display device which uses organic EL elements or the like.
2. Description of the Related Art
Although cathode ray tubes (CRTs) have been a mainstream of the display devices conventionally, recently, a demand for flat display devices has been increasing as display devices which replace the cathode ray tubes. Particularly, an organic EL (Electro Luminescence) display device which uses organic EL elements such as OLED (Organic Light Emitting Diodes) possesses excellent properties such as small power consumption, light weight, small thickness, excellent motion picture characteristics, and a favorable viewing angle. The development of the organic EL display devices has been in progress and some organic EL display devices have been put into practice.
These organic EL display devices are described in JP-A-2003-5703 (patent document 1), JP-A-2005-316110 (patent document 2) and JP-A-2001-92413 (patent document 3).
Patent document 1 discloses that a triangular waveform signal is used as a PWM signal within a light emission period, and the triangular waveform signal can be modified in various shapes. For example, according to patent document 1, to prevent the time-based center of gravity of emitted light from being dependent on light emitting grayscales, visual characteristics which differ from each other are acquired using a laterally symmetrical triangular waveform, an asymmetrical triangular waveform, a non-linear triangular waveform corresponding to gamma characteristic modulation, a plurality of triangular waveforms and the like.
Further, patent document 2 discloses an electro-optical device which includes a selection circuit for selecting one brightness display mode from a plurality of brightness display modes based on a result of determination made by a determination circuit. According to patent document 2, a gamma curve is suitably changed by adjusting a reference potential such as a gamma correction voltage when crushing occurs in a low brightness portion so that the grayscale difference can be easily distinguished in the low brightness portion. Patent document 2 also discloses that such processing is automatically executed by incorporating resistance capable of changing over a gamma reference power source circuit between a connection state and a disconnection state in response to a control signal and, at the same time, by programming the gamma reference power source circuit such that a main control circuit outputs a preceding control signal to lower the maximum brightness. Alternatively, patent document 2 further discloses that preceding processing may be executed corresponding to an input operation carried out by a user.
Further, respective organic EL elements which emit lights of red, green and blue which are used in an active-matrix-type organic EL display device (AM-OLED) in general differ from each other with respect to light emitting characteristics (light emitting brightness, a voltage-current characteristic, a voltage-light emitting brightness characteristic and the like) for respective colors. Such irregularities in the light emitting characteristics of the respective organic EL elements of red, green and blue also appear as the above-mentioned minute irregularities on a display screen.
To suppress the non-uniformity in display attributed to the irregularities in the light emitting characteristics of the respective organic EL elements of red, green and blue, for example, the above-mentioned patent document 3 discloses a technique which provides memories which perform gamma correction with respect to respective video signals of R, G, B which are supplied to respective organic EL elements of red, green and blue and changes gamma correction values for respective R, G, B.